Find users of a material / texture / image which is not displayed in the outliner
You can set your outliner to Blender File mode. This will show even datablocks with no users. Under materials you will find a list of all materials in alphabetical order. You can even search for datablocks by name in the small search field at the top:


To find users of a datablock, perform the same search in All Scenes mode instead of Blender File mode.